Cardiac Physiology

Part 1 Info

Circulatory System purpose

• transport O2 and nutrients to tissues

• removal of CO2 wastes from tissues

• regulation of body temperature

Circulatory system work with?

works with the pulmonary system: cardiopulmonary or cardiorespiratory system

Adjustments of blood flow during exercise of circulatory system

• increased cardiac output

• redistribution of blood flow from inactive organs to active muscle

Components of Circulatory System

heart, arteries/arterioles, capillaries, and veins/venules

heart

creates pressure to pump blood

arteries and arterioles

carry blood away from the heart

capillaries

exchange of O2, CO2, and nutrients with tissues 

veins and venules

carry blood toward the heart

Structure of the heart

cardiac tissue

• striated 

• branched

• many mitochondria

• involuntary control 

• connected through intercalated discs: purpose is to carry electrical signals

• spiral orientation: purpose is to squeeze blood up and out of ventricles 

Cardiac muscle structural comparison

contractile proteins: present

shape of muscle fibers: branching; shorter than muscle fibers

nuclei: single

Z discs: present

cellular junctions: intercalated discs

connective: endomysium

skeletal muscle structural comparison

contractile proteins: present

shape of muscle fibers: elongated

nuclei: multiple

Z discs: present

cellular junctions: no junctional complexes

connective: epimysium, perimysium, and endomysium

cardiac muscle functional comparison

energy production: primarily aerobic

calcium source: sacroplasmic and extracellular calcium

neural control: involuntary

regeneration potential: none; no satellite cells present

skeletal muscle functional comparison

energy production: aerobic and anaerobic

calcium source: sarcoplasmic retriculum

neural control: voluntary

regeneration potential: some possibility via satellite cells

epicardium/visceral pericardium characteristics

serous membrane including capillaries, lymph capillaries, nerve fibers

epicardium/visceral pericardium function 

lubricating outer covering 

myocardium characteristics

cardiac muscle tissue separated by connective tissues and including capillaries, lymph capillaries, nerve fibers

myocardium function

provides muscular contraction to eject blood from heart chambers

endocardium characteristics

endothelial tissue and thick subendothelial layer of elastic and collagenous fibers

endocardium function

serves as protective inner lining of chamber and values

myocardium

receives blood supply via coronary arteries

• high demand for oxygen and nutrients

main coronary arteries (left and right)

left main coronary artery

left ventricle and left atrium

• left anterior descending artery (LAD): front of left side of heart and septum

• left circumflex artery: lateral and posterior heart wall

right coronary artery

right ventricle, right atrium, SA node, AV node

• posterior descending artery: inferior aspect of the heart

• acute marginal artery: lateral portion of right ventricle & septum of the heart

atherosclerosis

progressive condition resulting in narrowing of arteries due to fatty plaque build up in the inner wall of an artery

-decrease in radius of the vessel results in decrease in blood flow

—myocardial ischemia

Myocardial infarction (MI)

plaque in coronary artery ruptures

triggers a blood clot which blocks blood flow downstream

blockage in coronary blood flow results in cell damage 

exercise is cardioprotective against MI

reduce incidence

improved survival

reduces the amount of myocardial damage from MI

-improvements in heart’s antioxidant capacity

-improved function of ATP-sensitive potassium channels

Cardiac Cycle

Systole (contraction phase and ejection of blood), diastole (relaxation phase and filling with blood)

at rest, diastole longer than systole

during exercise, both systole and diastole are shorter

systole pressure changes 

pressure in ventricles rises

blood ejected in pulmonary and systemic circulation 

-semilunar valves open when ventricular P > aortic P 

diastole pressure changes 

pressure in ventricles is low 

filling with blood from atria

-AV valves open when ventricular P < atrial P 

heart sounds

First (S1): closing of AV valves

Second (S2): closing of aortic and pulmonary valves

Cardiac Cycle at Rest and During Exercise

electrical activity of the heart

contraction of the heart depends on electrical stimulation of the myocardium

conduction system

conduction system

SA node: pacemaker, initiates depolarization

AV node: passes depolarization to ventricles; brief delay to allow for ventricular filling

bundle branches: to left and right ventricle

punkinje fibers: throughout ventricle

Electrocardiogram (ECG)

records the electrical activity of the heart 

-p wave: atrial depolarization

-QRS complex: ventricular depolarization and atrial repolarization 

-T wave: ventricular repolarization 

ECG abnormalities may indicate coronary heart disease (ST-segment depression can indicate MI)

Diagnostic Use of the ECG During exercise

-graded exercise test to evaluate cardiac function: observe ECG during exercise and also observe changes in blood pressure

-atherosclerosis

-ST segment depression or inverted T wave: suggests MI

atherosclerosis

fatty plaque that narrows coronary arteries

reduces blood flow to myocardium: (MI)

neural innervation of the heart 

parasympathetic nervous system: vagus nerve; slow HR by inhibiting SA and AV node 

sympathetic nervous system: cardiac accelerator nerves, increases HR by stimulating SA node and AV node, innervates the ventricles 

Regulation of Heart Rate

low resting HR due to parasympathetic tone

increase in HR at onset of exercise

• initial increase up to ~100 beats/min due to parasympathetic withdrawal - “ease off the brake”

• later increase >100 beats/min due to increased sympathetic outflow - “press on the gas”

Beta Blockers

beta-adrenergic blocking drugs

• complete with epinephrine and norepinephrine for beta adrenergic receptors in the heart (block)

• reduce heart rate and contractility (lower the myocardial oxygen demand)

prescribed for patients with coronary artery disease and hypertension

will lower HR during submaximal and maximal exercise

• important for exercise prescription

Heart rate variability (HRV) 

time between heart beats (standard deviation of R-R interval; measured by ECG or specialized equipment) 

indicator of sympathovagal balance: balance between SNS and PNS (factors affecting HRV: age and conditions affecting ANS

interpretation: wide variation in HRV is considered health; low HRV is a predictor of cardiovascular morbidity and mortality in patients with existing CVD

aerobic exercise can improve HRV

Cardiac output

amount of blood pumped by the heart each minute

product of heart rate and stroke volume (HR and SV)

• CO = HR x SV

depends on training state and sex

Regulation of stroke volume

end-diastolic volume (EDV): volume of blood in the ventricles at the end of diastole (preload)

average aortic blood pressure: pressure the heart must pump against to eject blood (afterload); mean arterial pressure

strength of the ventricular contraction (contractility)

End-diastolic volume 

cardiovascular system is a closed system, how would EDV change to meet metabolic demand? 

• dependent on venous return

• venous return increased by: venoconstriction, skeletal muscle pump, and respiratory pump 

Frank-starling mechanism

venoconstriction

SNS

skeletal muscle pump

rhythmic skeletal muscle contractions force blood in the extremities toward the heart re

respiratory pump

changes in thoracic pressure pull blood toward heart 

frank-starling mechanism

greater EDV results in a more forceful contraction

• due to stretch of ventricles

effects of sympathetics stimulation on stroke volume

Cardiac Ca2+ Handling with Sympathetic Stimulation

Factors that regulate cardiac output 

Cardiac Rate: parasympathetic and sympathetic nerves

Stroke Volume: sympathetic nerves (contractions strength); Frank-starling (stretch → contraction strength), end-diastolic volume and means arterial pressure